Hi,
The 40' long stock cars (5900s) used the larger Andrews trucks whereas the 5500-5849 stock cars used the smaller arch bar trucks (same as used under the 3000 series box cars and several series of flat cars and gons).
The Andrews trucks were used on many of composite steel/wood the idler flats built in the 1950's for the oil boom.
Although it may have been before the NGDF changed BBS software, I think someone posted a notice from the D&RGW calling in 5900 series stock cars for scrapping so the trucks could be used for the flats being built in Alamosa.
As I recall, the train crews did not like the longer 5900 series stock cars since the cattle chutes along the narrow gauge were set up for the 30' length stock cars. Whereas on positioning of a 5500 series stock car would allow two to be loaded from double ramp stock pens, the 5900s had to have each car positioned one at a time and this required longer loading times.
I am sure I have something not 100% correct. Please feel free to add of correct my summary.
Doug vV